The present invention relates to a kaolin composition exhibiting high bulking properties and to a method of producing the kaolin composition by beneficiating a kaolin clay. More specifically, the present invention relates to a beneficiated kaolin product having improved opacifying efficiency thereby rendering the product functional as a high performance paper filler, and to a method of producing the beneficiated kaolin product from a base kaolin composition having a brightness in the range of 89 to 91 and a particle size distribution of 88 to 92 percent by weight of particles less than 2 microns in equivalent spherical diameter.
It is well known in the paper industry, that a wide variety of fillers and pigments, such as titanium dioxide, calcium carbonate, silica, alumina and kaolin, are suitable for use as paper fillers. Kaolin, a hydrated aluminum silicate, is presently the most widely utilized and is available in a range of particle sizes and brightnesses, as well as being either delaminated or non-delaminated. Hydrated kaolin is white in color, has a fine particle size, and is relatively chemically inert, in addition to its low cost makes it an ideal paper filler. Although, calcined (anhydrous) kaolin is also available for use as a paper filler and can impart greater opacity to paper than the hydrated kaolin, it has the serious disadvantage of being more abrasive.
Prior art kaolin paper fillers are typically produced by a beneficiation process which typically consists of fractionating in a continuous centrifuge to control particle size followed by leaching to remove iron-based colored compounds. In the leaching process the kaolin is acidified with H.sub.2 SO.sub.4 to a pH of 3.0 to solubilize the iron. Sodium hydrosulfite is then added to reduce the iron to a more soluble ferrous form which is removed during the dewatering process. The flocculated clay, generally at approximately 30% solids by weight, is then filtered, such as by dewatering on a rotary vacuum filter to a solids level approximately 60% by weight. The filter cake is then either dried or redispersed with additional dry clay if it is to be sold as approximately 70% by weight solids slurry. To produce high brightness products, i.e., fillers having a brightness index greater than 90, impurities may be removed from the kaolin clay by further processing the kaolin clay through flotation or magnetic separation. To produce a delaminated product, the coarse fraction from the initial centrifugation is ground in sand grinders to shear the stacks of platelets normally found in kaolin and thereby produce individual particles having an equivalent spherical diameter less than 2 microns.
The anhydrous kaolin products generally available as paper fillers are typically produced by calcining hydrated kaolin at temperatures up to 1050.degree. C. so that structural hydroxyl groups are driven out as water vapor. The resulting material has an amorphous structure which contains voids which produce interfaces between kaolin and air. These interfaces of kaolin and air, which are not found in hydrated kaolin, serve as sites for light scattering. From the Fresnel Equation: ##EQU1## where Na=refractive index of component a
Nb=refractive index of component b
it is seen that the greater the difference in the refractive index of the various constituents of a system, the greater the reflection. Thus, the incorporation of air (N=1) into a paper fiber (N=1.5) and kaolin (N=1.56) matrix enhances light scattering. Because of this phenomena, calcined kaolin clay has greater optical efficiency than other kaolin products when utilized as a filler for paper.
For example, a paper handsheet formed from a paper pulp incorporating a commercially available calcined (anhydrous) kaolin filler at a level of 10 percent by weight exhibited an opacity of 80.9, a brightness of 87.0, and a scattering coefficienct of 2030 sq. centimeters/gram. For comparison, a paper handsheet formed from a paper pulp incorporating a commercially available delaminated hydrated kaolin filler at a level of 10 percent by weight exhibited an opacity of 76.4, a brightness of 84.8, and a scattering coefficienct of 1180 sq. centimeters/gram. The greater brightness of the paper handsheet incorporating the calcined kaolin filler is largely attributable to the greater brightness of calcined kaolin, typically about 93, vis-a-vis the less brightness of hydrated kaolin, typically about 90.
Other opacifying pigments are commercially available to the papermaker. Because of its high refractive index, 2.55 for anatase and 2.7 for rutile, titanium dioxide is presently the opacifier of primary commercial importance. When incorporated into paper, titanium dioxide also imparts exceptional brightness and whiteness to the sheet. However, the main disadvantage of titania is its cost. Commercial grade titania is approximately 4 times more costly than commercial grade anhydrous kaolin and up to 25 times higher than the commercial grade hydrated kaolin. Due to this cost factor, other products have been developed and are in commercial use of titania "extender" pigments. These products, which can be used to replace portions of the titanium dioxide without a loss of opacity of the paper, include calcined clay, delaminated hydrated clays, fine-particle size silica and alumina, and sodium aluminum-silicate. The effectiveness of either calcined or delaminated kaolin clay as extenders for titanium dioxide can, in part, be attributed to the paucity of colloidal fines, i.e., particles having a fineness of less than about 0.3 microns equivalent spherical diameter. Calcined kaolin can be produced having a content of only 5 to 10% by weight of colloidal particles, and standard filler clay can be produced having a content as high as about 40% by weight of colloidal particles.
The influence of defining upon the optical performance of kaolin pigments has been previously observed. "Defining" refers to the operation of separating and discarding a percentage of the fine fraction of the kaolin suspension. U.S. Pat. No. 3,085,894 discloses the selective fractionation of natural occurring kaolin by sedimentation and centrifugation into several size ranges to produce a coating pigment of improved opacity, brightness and gloss. As discussed therein, the optimum optical performance was obtained with a kaolin product having a particle size distribution in the range of 70% to 84% of the particles less than one micron in equivalent spherical diameter.
It has also been shown that surface treatment of kaolin particles can improve the optical performance of kaolin pigments. For example, in U.S. Pat. No. 4,076,548 a high bulking clay was produced by selective flocculation of ultra-fine particles of kaolinite with the addition of either an organic flocculant, or a polyamine and citric acid, or ethylenediamine and citric acid. As a result of such treatment, voids are incorporated into the pigment which result in better opacity and improved brightness and reflectance in paper coating. Although this product has been commercialized by Georgia Kaolin Company, Inc. of Union, N. J., under the trade name of Astra-Lite, it has not been found suitable as a paper filler because the aggregate structure breaks down under the shear conditions inherent in the wet-end of a paper machine. Subsequent losses in scattering efficiency occur as a result of shear. In U.S. Pat. No. 4,640,716, it is disclosed that the addition of the zirconium ion to an uncalcined clay results in a high bulking pigment having a higher scattering coefficient than an untreated base. When incorporated into a fiber matrix, however the product did not result in improved opacity over the product incorporating its unmodified counterpart.
Accordingly, it is an object of the present invention to provide a hydrated kaolin product which exhibits improved opacifying characteristics as a paper filler thereby being suitable as a substitute or extender for commercially available calcined kaolin and titania paper fillers.
It is a further object of the present invention to provide a method of producing a hydrated kaolin product exhibiting improved performance as a paper filler.